One already knows dual stack sensor MEMS assemblies, wherein the sensing die comprises a Pyrex® pedestal supporting a sensing diaphragm, mounted on a metal, ceramic or plastic base.
Such a dual stack pressure sensing die is illustrated on FIG. 1 with a pressure sensing structure 1 and a pedestal 2.
Pressure sensing structure 1 generally consists of a bulk micro machined diaphragm 5 supported by rigid outer frames 7, said diaphragm 5 and said frames 7 being formed on a silicon wafer by chemical etching. Strain gages, or piezoresistors, 6 are diffused, implanted or deposited on diaphragm 5 for sensing the deflection produced in diaphragm 5 by applied pressure. Pedestal 2 may be made of a Pyrex wafer which functions as a pedestal for supporting pressure sensing structure 1. For absolute (PSIA) pressures, pedestal 2 has no vent hole whereas a vent hole 14 is provided on pedestal 2 for gage (PSIG) and differential (PSID) pressures.
The two wafers 1, 2 are anodic bonded and are sawed to form a dual stack pressure sensing die of a type which is commonly used in the piezoresistive pressure sensor industry. Pyrex pedestal 2 of the dual stack pressure sensing die is attached with an elastic adhesive 8, such as RTV, to a metal or ceramic base 3, said adhesive and said base both having a thermal coefficient of expansion larger than that of Pyrex glass. The RTV must be cured at a high temperature.
Considerable “die-attach” compression stresses are locked-in the RTV adhesive during the curing process.
The stress deforms the Pyrex pedestal 2 and the outer frame 7 of the silicon pressure sensing structure 1, which in turn deflects the pressure sensing diaphragm 5. The piezoresistors sense the diaphragm 5 deflection and produce a residual null offset output that is proportional to the locked in die-attach stresses.
Pressure sensors are customarily required to operate in a wide range of temperatures.
However, the temperature variations increase and decrease the locked in die-attach stress in the RTV adhesive.
And, since RTV is not perfectly elastic, the compression stress locked in the RTV does not return perfectly to its original value after each temperature cycle involving temperature hysteresis.
This temperature hysteresis is the main source of short term instability and drift of sensor offset. In addition, the effects of continuing bond relaxation and RTV aging are the source for long term deterioration of die-attach stresses and produce long term sensor drift.
As a result, pressure sensors employing conventional pedestals suffer from short and long term drift problems, which are exacerbated in low pressure range sensors that employ thin diaphragms.
Earlier attempts to isolate the die-attach drift problem involved etching slots or channels in the silicon die around the sensing diaphragm (see US 2001/0001550 A1) or adding a plurality of relief channels etched in an upper and a lower surface of an intermediate layer (see U.S. Pat. No. 6,822,318 B2).
The use of slots or channels as stress isolators requires them to be relatively flexible.
Since their geometry is dictated by the limited thickness of silicon wafer used in MEMS sensors, this approach requires multiple upper and a lower surface channels is slots with very thin webs, which is problematical and costly to implement.
Furthermore, etching slots or channels in the silicon die around the sensing diaphragm in close proximity to the piezoresistors may create stability problems.